1. Field of the Invention
The present invention relates to an NO.sub.x analyzer for quantitatively determining nitrogen oxide (hereinafter referred to as NO.sub.x) components contained in an exhaust gas from, for example, a diesel engine and the like. In particular, the present invention relates to a chemical luminescence-type NO.sub.x analyzer adapted to introduce a sample gas into an NO detector through an NO.sub.x convertor for reducing NO.sub.x to NO.
2. Description of the Prior Art
In the so-called chemical luminescence-type of NO.sub.x analyzer, NO acts upon ozone to emit a light. The quantity of emitted light has been detected to quantitatively determine NO.sub.x components in a sample gas. The NO.sub.x components in the sample gas are first reduced to NO by means of an NO.sub.x convertor, and then are introduced into a reaction tank for reacting the resulting NO with ozone.
However, in the case where aromatic hydrocarbons, such as xylene, are contained in the sample gas, then when the sample gas is heated to about 220.degree. C. in a heater incorporated in the NO.sub.x convertor, the aromatic hydrocarbons are oxidized to be turned into phthalic acid anhydride. Because phthalic acid anhydride has a high saturated vapor pressure at high temperatures of about 200.degree. C., or more, it exists in the gaseous form at these temperatures. However, the phthalic acid anhydride is apt to sedimentate in the solid form at lower and normal temperatures. Because such solidified phthalic acid anhydride is apt to clog a capillary arranged immediately before the NO detector, and thus to lower the accuracy of measurement provided by the NO.sub.x analyzer, a filter has been arranged in the vicinity of a sample gas exhaust port of the NO.sub.x convertor. That is, conventionally a filter may be installed between the NO.sub.x convertor and the NO detector.
However, in the case where solidified phthalic acid anhydride is removed by such a filter, as above described, difficulties have occurred. These difficulties include, for example, a necessity for frequent maintenance operations to clean, exchange, and to otherwise service the filter. These frequent maintenance operations cause increased idle time and increased operating costs for the NO.sub.x analyzer.